the bar chart has this limitation :
you can either use 2 dimensions and 1 measure or 1 dimension and as many measures you want.
you have 2 options:
1 concatenate the two tables in the load script :
'Others' as [Asset class]
[Total Amount] as Amount
Then you'll have a simple chart :
1st dimension: Month
2nd dimension: Asset class
your second option is to leave the table as they are and to build this chart.
1st measure: sum(if([Asset class]='A', Amount)
2nd measure: sum(if([Asset class]='B', Amount)
3rd measure: sum([Total Amount])